In recent years, a technique for performing time synchronization between communication devices has attracted attention. An example of a technical specification for this time synchronization technique is IEEE (Institute of Electrical and Electronic Engineers) 1588. The IEEE 1588 defines PTP (Precision Time Protocol). This PTP is a protocol for accurate time synchronization between communication devices within a network.
An outline of a time synchronization process using the PTP will be described below. In the time synchronization, a propagation delay time between a master node and a slave node is measured and time information within each device is corrected based on the propagation delay time, thereby achieving accurate time synchronization.
First, the master node transmits a Sync message. At this time, the master node holds a time t1 when the Sync message is transmitted. The slave node holds a time t2 when the Sync message is received. The master node inserts information about the time t1 into a Follow_Up message, and transmits the Follow_Up message. The slave node receives the Follow_Up message to thereby obtain the information about the time t1. Subsequently, the slave node transmits a Delay_Req message at a time t3. The master node inserts, into a Delay_Resp message, information about a time t4 when the Delay_Req message is received, and transmits the Delay_Resp message to the slave node. The slave node receives the Delay_Resp message to thereby recognize the times t1 to t4. A propagation delay time can be calculated by the formula ((t2−t1)+(t4−t3))/2. The slave node corrects the time of the slave node using the propagation delay time, thereby performing time synchronization between the slave node and the master node.
In this case, relay devices are generally present on a path between the master node and a client node. Each relay device measures a processing delay time within the relay device, and sets the processing delay time in a correction field within each synchronization message (e.g., Follow_Up message). The term “correction field” refers to a field (delay amount field) in which a total time during which the synchronization message remains in nodes (e.g., relay devices) other than the master node and the slave node (i.e., a total delay time in all devices except the master node and the slave node) is set.
In general, each relay device modifies the correction field every time processing in an internal processing unit in which a delay occurs is completed. Patent Literature 1 discloses one mode of time control using a TSF (Timing Synchronization Function) timer.